The present invention relates to a light quantity adjusting method which is carried out in advance when a light quantity level of emitted light from a light emitting device having a light emitting element such as a laser light source is changed over to another one, and a light quantity adjusting apparatus using the light quantity adjusting method, and also relates to an image forming apparatus for forming an image on a recording material using a laser light source or the like, to which the light quantity adjusting apparatus is applied. In particular, the present invention relates to an image exposure technique for, when a recording material for printing such as a thermal plate is exposed, determining a normal exposure power and an exposure power for an edge.
In the field of printing, there is generally used an image forming apparatus for focusing a laser beam on a recording material such as a photosensitive material as a recording material for printing to expose the recording material. For example, a Computer to Plate (CTP) process has been carried out, in which a digital image is formed using a computer, and during plate-making, an image is directly recorded on a printing plate without interposing a film. In the image forming apparatus used in such a CTP process or the like, from a viewpoint of a productive efficiency with which an image is formed on a recording material for a short time period, there is used a scanning system in which an image is formed with a laser beam while the recording material is relatively moved at a high speed with respect to the laser beam emitted from a laser light source. For example, a photosensitive material is exposed using the laser beam in correspondence to a digital image signal to record an image on the photosensitive material while the photosensitive material is held on a surface of a rotating drum.
With this technique, a lighting state of a light beam emitted from a light source for exposure is controlled using a binary coded image signal generated based on image data of an original image to be recorded, and a photosensitive material is two-dimensionally scanned with the light beam to record a desired image on the photosensitive material by moving the photosensitive material relative to the light source for exposure. The image for plate-making is a dot image using dots. The dot image is recorded in the form of a set of a large number of dots formed through scanning exposure using a light beam having a predetermined size in correspondence to the resolution.
When a single dot or a fine pattern such as a fine line is exposed using a short pulse in forming an image using this image forming apparatus, there arises a problem that since the exposure recording is carried out while the recording material is moved at a high speed, the exposure power (exposure energy density) corresponding to this movement is insufficient, and thus the fine pattern is not precisely exposed and recorded by a light beam. For example, when a negative photosensitive material as a recording material is exposed, photo-curing of an exposed portion of the negative photosensitive material does not sufficiently progress owing to insufficiency of the exposure power, and hence the dot, the fine line or the like is not precisely reproduced. In addition, when a positive photosensitive material as the recording material is exposed, the exposed portion of the positive photosensitive material such as the dot or the fine line, which becomes a non-image portion is not perfectly removed owing to the insufficiency of the exposure power.
Heretofore, the beam diameter of the laser beam is reduced and also the exposure power density is increased to cope with the above-mentioned problem. However, such methods cause another problem that an optical system is contaminated due to the ablation and the life-time of a semiconductor laser diode is shortened since the exposure power is increased. For this reason, the precise processing cannot be still executed for the fine pattern, and the life-time of the laser light source is shortened to increase the maintenance cost.
On the other hand, JP 8-23422 A discloses an image forming apparatus in which when turn-ON/OFF of emission of a laser beam is switched in correspondence to a fine pattern, a light quantity level of the emitted laser beam for a limited time period of at least one of a time period right after turn-ON and a time period right before turn-OFF is made higher than that for a time period other than that limited time period to allow an exposure recording edge (edge-portion) of a fine pattern to be precisely exposed to the laser beam. That is to say, there is disclosed a system in which when an internal portion except for an edge portion is exposed, the light quantity level of the laser beam is set to a predetermined level and the exposure power is set to a normal exposure power, while when a dot of the edge portion of the image is exposed, the light quantity level of the laser beam is increased and the exposure power is set to a dot exposure power. That is, there is disclosed a so-called method of increasing a dot light quantity in which only for a certain time period of one or both of a time period right after rise of the pulse and a time period right before fall of the pulse, the exposure is carried out at a power higher than that in the normal exposure.
However, in the system disclosed in JP 8-23422 A, since the exposure power is changed between two stages, i.e., the normal exposure power and the dot exposure power, the light quantity of the laser beam must be adjusted to two levels of high light quantity level and low light quantity level. This requires the complicated work in which prior to the processing for, forming an image on the recording material, the adjustment must be carried out twice, i.e., the light quantity of the laser beam must be adjusted to the high light quantity level and the low light quantity level. The light quantity adjustment must be carried out whenever the exposure recording is carried out, and whenever the recording material or a kind of recording material is changed over to another one. Thus, there arises a problem that when a plurality of recording materials are processed in large quantities, a total time period required to adjust the light quantity becomes long, and hence the light quantity adjustment process is inferior in productive efficiency with which an image is recorded on a recording material for a short time period.